Tristimulus photometer



Oct. 18, 1955 G. c. szlKLAx 2,720,811

TRISTIMULUS PHOTOMETER Filed Sept. 19, 1950 aa WAVE EA/67%'l INVENTOR Ee urge E. Ezikla 4a@ fao 600 WAL/' LENGTH ATTORN EY United States Patent O TRISHMULUS PHoToME'mR George C. Sziklai, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 19, 1950, Serial 185,591

6 Claims. (Cl. 88-14) This invention relates to improvements in photoelectric colon'meters, and particularly to an improved photoelectric colorimeter of the so-called tristimulus type.

At one time, the only known way of comparing colors was by visual observation. Although the human eye theoretically is adapted to distinguish very slight variations in color, it is well known that subjective factors cause dilferent individuals to see the same colors differently, and may even cause the same individual to see the same color differently at dilerent times; For this reason, many attempts have been made to develop color measuring instruments with which reproducible color measurements could be made. Of course, with such an instrument available, it would also be possible to define any measured or analyzed color by reference to some arbitrary standard, since the measurements obtained with the instrument could be referred directly to the standard.

One instrument that provides an exact analysis of the color of a light is the spectrophotometer, which records the intensity distribution of the light over the whole visible spectrum. The result obtained however is limited in usefulness for two reasons:

l. The color is specified by a large number of coordinates. If the number of these coordinates is reduced, the precision of the color information is reduced as well. It is customary to specify spectrophotometric data in the 40G-700 millimicron region with an intensity given at least every 10 millimicrons, thus requiring at least 300 sets of data.

2. Actually two lights having different spectral distiibutions may look alike. This principle is used extensively in color reproducing processes such as photoggraphy, television, etc. For these processes a spectrophotometric curve by itself is not readily suitable for colorimetric matching.

One of the methods that has been devised for specifying a given color in terms of an arbitrary standard is to specify that additive combination of three primaries which will reproduce the given color. This method is known as tristimulus specication, and is directly involved in the present invention.

rl`he spectral distribution of the three primaries most universally used for tristimulus color specification is that of the so-called I. C. I. standard observer (see D. B. Judd The 1931 I. C. I. Standard `Observer and Coordinate System for Colon'metry, Journal Optical Society of America, vol. 23, pg. 359-1933). Having analyzed a color to the extent of breaking it down into its spectral distribution, it becomes expedient to further simplify the specification by utilizing two coordinates according to the following relations:

(i) X=fRedi (a) Y=fagdx 3) z=fRazx Z,7Z0,8 il Patented Get. 18, 1955 Vice Y (5) y XH/Jfz where R is the spectral response of the color sample being analyzed, x, y, z are the three distribution factors of the standard I. C. l. observer, and x and y are the desired coordinates. With the specification reduced to two coordinates, the color can be specified on a twodimensional chart or graph.

In order to avoid the tedious integration procedure indicated by Equations 1-3 above, a proposal was made to construct an instrument having three photocell-lter combinations with responses corresponding to the spectral distribution factors referred to above (see Twyman et al., British Patent 324,351). However, with such instruments as have heretofore been developed in accordance with this proposal, the extent of approximation of the desired response is limited. Particularly, the approximation is seriously limited for the 'r curve which, as will be shown hereinafter, is extremely diiiicult to simulate with a single photocell-lter combination. The problem of simulation is further complicated when the color source is not a reflector or transmitter so that a special illuminant cannot be used.

It is a general object of the present invention to provide an improved tristimulus photometer for coloi` analysis.

A more specic object of the invention is to provide a simple tristimulus colorirneter in which the spectral distribution factors of the I. C. I. standard observer are closely approximated.

In accordance with the invention, the foregoing and other related objects and advantages are attained by providing means in a tristimulus colorimeter for obtaining the three integrated tristimulus values simultaneously from three combination light filters and photosensitive elements approximating the spectral distribution curves of the l. C. I. standard observer and combining the resultant signals in such a manner as to improve the original approximations. As will be shown, a system of this type is capable of approximating closely the desired overall response.

A more complete understanding of the invention can be had by reference to the following description of an illustrative embodiment thereof, when considered in connection with the accompanying drawing, wherein:

Figure l is a graph showing the tristimulus specications of the various parts of the spectrum according to the I. C. I. standard vobserver system,

Figure 2 is a schematic diagram of a tristimulus photoelectric colorirneter in accordance with the invention, and

Figure 3 is a graph showing the spectral response of the apparatus of Figure 2.

As was previously stated, analysis of a color according to the tristimulus specification method requires that one determine the spectral distribution of the energy in that color, and then integrate Equations 1 3, above, with the spectral response information inserted, to allow determination of the coordinate values x and y. If one attempts to by-pass the integration step in order to obtain x, y and z directly by using three lter-photocell combinations corresponding to the three curves of the standard observer factors-JE, 3; and?, one immediately is faced with the diihculty of designing a filter which will duplicate the x curve. This can best be seen by reference to Fig. 1,

' wherein the tristimulus values are plotted against wavelength for the factors x, y and z. As shown in Fig. l, the x curve is double looped, having maximum values in the vicinity of 440 and 600 millimicrons, and a minimum value in the vicinity of 500 millimicrons. While it is relatively simple to design a light lter having transmission characteristics which, in combination with the spectral sensitivity of certain photocells, will closely approxigrads-1r 3; mate thel single loop 5f' and 'zcurves, the double looped; characteristic cannot readily be designed into a lterphotocell combination. However, it is to be noted that the lower wavelength loop inthe .x curve appears some:-V what like an attenuatedY version of the z curve. In aclcord a nce wi tl1 the presentVv invention, this;rel`ationbetween the x: and: z curves is taken advantage of toA provide' a relatively simple yet eiiicient colorimeter..

Referring to Fig. 2V of they drawing, ay colorimeterl arranged in accordancewiththe invention comprises. three photocell-ilter combinations 10, 12;, 14 arrangedf to receive light'v from; a color samplev to. ber .analyzed (not shown) throughrk ay crossed semi-transparent mirror 16... i

The;lightlreachingy the mirror 16, may originate at asourcez (not shown-)e and acquire; the spectral distribution of the. samplezeithen by reectionfrom. the surface. thereof onzby. transmission therethrough. Alternatively the. sample;

itself may constitute the light source., as in; theA case ofthe:Y multicolor fluorescent screens utilized in certain*l types;v of;

color television apparatus. All` that. is required is alight beam` containing. the spectral components of. the color sampleto be. analyzed.

Since the photocell-iilter combinations 10,. 12,. 14 inv Ythe apparatus are similar in conguration, only one of thel combinations, 10, has been shown in'. detail. The combination preferably includes a photosensitive, element 18V of theso-called photomultiplier type, such as. a commercial typeY lP22 phototube. The photocell 18. comprises a cathodey 20, a plurality of secondary electron emitters or dynodes 22a-221', and an anode 24. Asis well known, the cathode 20 is adaptedV to emit electrons in response to light impinging thereon, and the, dynodes 22a-22i are adapted to emit secondary electrons` when bombarded'with primary electrons. Thus, in the tube 18, when light impinges on the cathode 20, electrons emitted therefrom will bombard the iirst dynode 22a. Secondary electrons willV leave the rst dynode 22a in an amount exceedingv the number of bombarding primary electrons, and will strike the second dynode 22'b. This eiect will continue cumulatively through the succeeding dynodes 22e-22'i, so that a relatively large current will flow t'o the anode 2.4' for a small current from the cathode 20'.

Dynode and' anode voltages for the tube 18 preferably are provided by a radio frequency oscillator power supply 26. The Ypower supply 26' includes a voltage doublerwhen. such. filters are utilized with photocells.Y must take into account the spectral response of the photocell. The lter-photocell combinations, together with their measurig devices, constitute separate signal channels for the x, y, and z components of the color being analyzed.

Assume, then, that one has selected photocell-lter combinations hav i ng chaiacteristics similar tothe curves for the factors y and z of Fig. l. These may, for example, be the combinations 10, 14 of Fig. 2. As previously stated', it: is practically impossible to obtain a photocell-lter combination with` the double loop characten'stic of the xV factor. in Fig. l. However, av combinat ion can be obtained which will satisfactorily cover the x factor for the regionbetween 560' andV 700 millimicrons. Assuming, then, that the combination 12 of Fig. 2 has such a spec tral response characteristic (i. e. corresponding to the x curve of Fig. 1 between 500 and 700 milli microns),v the; lower-frequency loopin the x curve is obtained;l in accordancewith :the invention, by providing. a cross-feed; circuit. between the. output c ircuits of the combinations: (12; andi 14;) which.y cover thex and z curves. To provide the. necessary: attenuation,4 a variable resistor 42 orl other.v comparable attenuator may beincluded in.V the.V cross-feed path. Also, some provision should-be made to ensure that there will. be feedthrough only from the z channeltothex channel, and not in the other direction. For example, the-cross-feed circuit may include a crystal rectifier.' elem ent 4.4 which will only permit current ilowing inthe z channel to reach, the meter in the x channeL. andV not vice-versa..

When the. apparatus, of Eig; 2 is. in operation, colored light from the sample. being4 tested will be directed. towardeach of theflter-photocell combinations 10, 12, 14.

If the colored. lightsample has componentsfalling withinV the response. characteristic of one or more ofthe. combinations 10, Il.,` 14,', then the. measuring device in the channel involved will'proyide an indicationI of thel integrated valueof the light withinthe limits of that channel. Also, if the light sample has components withinV the wavelength limitsy of the channel, then the output'of the channel will inclnde. both E and; attenuated components.

The. output-.signals may. be applied; to meters of' the proper sensitivity,.andffi1orrr the: readings,Y the I. C. I. co-

rectier sectionV 28 for converting alternating voltage from v theV usual commercial supply (not shown) to unidirectional voltage for energizing an oscillator section 30.

- Radio frequency voltage developed in the oscillator secis conventional, itis believed that detailed description of the-operation thereof is unnecessary. Y

The anode circuits for the photocells inthex combina-V tions 10, 12, 141include measuring device such'as microammetersf36, 38, 40, orthe like;

photocell. circuits, asV willl be described hereinafter.

Eachf. ofthe; photocell-iilter combinations:v 10.;` 12,.l 1.4; is: Y

selectedto have aV spectrala response corresponding as closely asi p2ssible to the spectral response curve of one ofthe x, y, z factors of Fig. l. Throughout this description thel term photocell-iilter combination hasbeen used for the reason that thejspectral response on anyfpgiven ordinates; carr'becomputed' easily by dividing X and` Y` with the: sum of the threereadngs..

Another application of the device may be theV use of two instruments, connected; to.y three; zero centermeters. OneV of, the instruments then, can; bef aimed' at* the original subject,` the othen'at. a reproducing device.A A zero indication on thethree; meters. then. would correspond` toav perfectmatch. for thezstandard observer, While Vpositive or y negative indications would provide a sense and magnitude These measuring devices provide an indication of the'currentiowing. in the f photo-tube, selectedV at random, may differ-somewhat from .l

thatof anotherl tube of the same-type. This simply means that althoughthe filtersthemselvesi can be designed to have a spectral. characteristic approximating one of the curves of Fig. l, or a portion thereof, the results obtained of the correctionsrequired For example, in aV color television studio.; one or? the: instruments might be set up to survey a' scene; beingY televised;` and the: other instrument setupffacingza'. monitorr receiver on whichthe televised picture is; displayed. Any; departure from zero reading on any of the three zero center meters would indicatey the necessary adjustmentstofbe made at the television camera for proper'color;reproduction; Y Y

Since, many; changes. could be-mad'erin ,the speciiic apparatusV shown and: described, Vall. within the scope and spirit ofA the inventiom the; foregoingl is toV be construedr asyillust-rative, and not in a limiting sense.

What is claimed is:

1-. InV apparatus for analyzingv the coloi of light, in combination,l a. plurality oflight filters each-having a` dif.- ferent light transmission characteristic, each of said characteristics approximatingat least a major Vportion of a` different one of the spectral distribution curves, respec-V tively, of the standard primary. colors defined by theInternational Commission @rr Illumination, a photosensitiveelement associated with each said filter and responsive to light passing through the lter associated therewith, each said element being connected in an electric circuit to control the current in each said circuit as a function of the light reaching the element in that circuit, measuring means connected to each said circuit to measure the effects of said elements on said currents, and unidirectionally conductive means connecting only one of said circuits to only one other of said circuits to furnish to said other circuit a current proportional to the current in said one circuit.

2. A tristimulus photometer comprising lirst, second and third light filters having different light transmission characteristics, each of said characteristics approximating at least a major portion of a diierent one of the spectral distribution curves, respectively, of the standard primary colors defined by the International Commission On Illumination, a plurality of photomultiplier tubes each associated with a separate one of said filters and responsive to light passing through said filters for generating an electric current proportional to the light received by said tubes through said filters, means to direct light to be analyzed to said tubes through said filters, a plurality of circuits each connected to a separate one of said tubes to conduct the currents generated by said tubes, measuring means connected in each said circuit to measure said currents, and an auxiliary circuit including a undirectionally conductive element connecting two of said tube circuits to transfer current from one to the other of said two circuits in an amount proportional to the current flowing in said one circuit.

3. Apparatus as defined in claim 2 wherein said auxiliary circuit comprises a serially connected rectiiier and variable resistor.

4. In a photometer apparatus of the type wherein light to be analyzed is segregated into three spectral components and converted by photocell-light iilter combinations having dilerent spectral responses to electric currents in circuits connected to said photocells, said responses approximating the spectral distribution curves y, E, and the major lobe of the curve x of the standard primary colors defined by the International Commission On Illumination, and wherein means are connected to said circuits to measure said three currents as a measure of said three spectral components, the ni'provement which comprises a single unidirectionally conductive circuit connecting the E spectral component measuring circuit to the 'X spectral component measuring circuit to provide in the last named circuit a current proportional to the current in said z spectral component circuit.

5. In apparatus for analyzing the color of light, in combination, a plurality of light filters each having a dilerent light transmission characteristic, a majority of said characteristics approximating a majority of the spectral distribution curves, respectively, of the standard primary colors defined by the International Commission On Illumination, and a minority of said characteristics approximating a major portion of the remaining spectral distribution curves, respectively, of said primary colors dened by said Commission, a photosensitive element associated withreach said filter and responsive to light passing through the ilter associated therewith, each said element being connected in an electric circuit to control the current in each said circuit, as a function of the light reaching the element in that circuit, measuring means connected to each said circuit to measure the elects of said elements on said currents, and unidirectionally conductive means connecting only one of said circuits to only one other of said circuits to furnish to said other circuit a current proportional to the current in said one circuit.

6. A tristimulus photometer comprising first, second and third light iilters having dilerent light transmission characteristics, two of said characteristics approximating the y and z' spectral distribution curves, respectively, of the standard primary colors defined by the International Commission On Illumination, the third characteristic approximating the major lobe of the twolobed 'x spectral distribution curve of said primary colors defined by said Commission, photomultiplier tubes associated one with each said filter and responsive to light passing through said filters for generating an electric current proportional to the light received by said tubes through said filters, means to direct light to be analyzed to said tubes through said filters, circuits connected one to each said tube to conduct the currents generated by said tubes, measuring means connected in each said circuit to measure said currents, and an auxiliary circuit including a unidirectionally conductive element connecting only two of said tube circuits to transfer current from one to the other of said two circuits in an amount proportional to the current flowing in said one circuit, said one circuit comprising the tube associated with the filter having the E characteristic, and said other of said two circuits comprising the tube associated with the filter having the characteristic.

References Cited in the le of this patent UNITED STATES PATENTS 2,413,706 Gunderson Jan. 7, 1947 2,483,452 Berkley Oct. 4, 1949 FOREIGN PATENTS 324,351 Great Britain Ian. 20, 1930 687,405 Germany Jan. 29, 1940 

